why rotors only crack on the outside??
10-01-2007, 07:20 PM
#2
Melting Slicks
the outside of the rotor cools more rapidly when you come off track. Its exposed to the cool ambient air, while the inside of the rotor has engine heat/ no convection of ambient air.
10-01-2007, 07:54 PM
#5
Drifting
Cracks are created as a result of the thermal and mechanical forces applied to the rotor and the rotors attempt to remain at its lowest surface energy state. A free surface interface (air-rotor) has less ability to support internal stress (force per unit area) or shear applied to the surface than does the interior mass.
10-01-2007, 08:04 PM
#6
Melting Slicks
10-01-2007, 08:05 PM
#7
Melting Slicks
Cracks are created as a result of the thermal and mechanical forces applied to the rotor and the rotors attempt to remain at its lowest surface energy state. A free surface interface (air-rotor) has less ability to support internal stress (force per unit area) or shear applied to the surface than does the interior mass.
DING DING DING !!!!! WE HAVE A WINNER !!!!
Exactly right, what is happening is the outer surface is attached to the hub area and that is (relatively) cool. The area above that is really hot, so it yields in compression at high temp (the inner part is holding in the hot surface, and since it is cooler it is stronger). When the disk cools, the outer area has shrunk, so now it goes into tension, and eventually cracks. Sincer there is no material to "fight" against on the inside, the inner surface doesn't yield in compression (as much) and you don't have the cyclic thermal stress. Big buck 2 piece rotors don't crack as easily since they get let the disk grow an each side equally by letting it slide outward on the pins....
10-01-2007, 08:17 PM
#8
Burning Brakes
Thread Starter
DING DING DING !!!!! WE HAVE A WINNER !!!!
Exactly right, what is happening is the outer surface is attached to the hub area and that is (relatively) cool. The area above that is really hot, so it yields in compression at high temp (the inner part is holding in the hot surface, and since it is cooler it is stronger). When the disk cools, the outer area has shrunk, so now it goes into tension, and eventually cracks. Sincer there is no material to "fight" against on the inside, the inner surface doesn't yield in compression (as much) and you don't have the cyclic thermal stress. Big buck 2 piece rotors don't crack as easily since they get let the disk grow an each side equally by letting it slide outward on the pins....
Exactly right, what is happening is the outer surface is attached to the hub area and that is (relatively) cool. The area above that is really hot, so it yields in compression at high temp (the inner part is holding in the hot surface, and since it is cooler it is stronger). When the disk cools, the outer area has shrunk, so now it goes into tension, and eventually cracks. Sincer there is no material to "fight" against on the inside, the inner surface doesn't yield in compression (as much) and you don't have the cyclic thermal stress. Big buck 2 piece rotors don't crack as easily since they get let the disk grow an each side equally by letting it slide outward on the pins....
cool// (excuse the pun) never really thought of that , thanks for the translation,,, for i am a simple man!!
10-01-2007, 08:32 PM
#9
Safety Car
how about the problem I noticed.
The cooling ducts are (sometimes) pointed at the inside face.
Since I realized this, I changed my ducts to point to the inner center hub so the veins can pull in the fresh air. Since I did this, I havn't cracked 1 rotor. I now have no direct air pointing at the inner face.
I also noticed some pads are just prone to crack rotors.
XP12's are very gentle on rotors, where as PF01's are more agressive on rotors.
The cooling ducts are (sometimes) pointed at the inside face.
Since I realized this, I changed my ducts to point to the inner center hub so the veins can pull in the fresh air. Since I did this, I havn't cracked 1 rotor. I now have no direct air pointing at the inner face.
I also noticed some pads are just prone to crack rotors.
XP12's are very gentle on rotors, where as PF01's are more agressive on rotors.
10-01-2007, 09:35 PM
#10
Melting Slicks
One thing I saw someone doing while I was at VIR. He had heavy cardboard box material cutout in a large circle. He leaned them against the front wheels in the pit area in the front only. I asked and he said it cut down on the cool air hitting the outside face.
I tried it on my last set of rotors and they lasted a bit longer. Hard to tell if that helps or I simply wasn't as rough on the rotors, different air temps, etc... Then again those rotors were used at CMP and that is more abusive than VIR. They haven't cracked yet. Blank NAPAs.
I tried it on my last set of rotors and they lasted a bit longer. Hard to tell if that helps or I simply wasn't as rough on the rotors, different air temps, etc... Then again those rotors were used at CMP and that is more abusive than VIR. They haven't cracked yet. Blank NAPAs.
10-01-2007, 10:33 PM
#11
Burning Brakes
Thread Starter
some of the guys i run with are testing a new system that combines a fan for non ducted cars that blowes air into the center of the hat, and is controled buy a temperature switch mounted to a plate on the spindle, the proposed name ... brake wind!
the point is the fan will run after you pull in the pits and cool the rotor evenly,,,
the point is the fan will run after you pull in the pits and cool the rotor evenly,,,
10-02-2007, 12:06 AM
#12
Tech Contributor
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The inside of the rotor will also crack. A couple of seasons ago one of my rear rotors cracked all the way through. The crack was big enough that you could see through it.
Bill
Bill
10-02-2007, 01:21 AM
#13
Drifting
how about the problem I noticed.
The cooling ducts are (sometimes) pointed at the inside face.
Since I realized this, I changed my ducts to point to the inner center hub so the veins can pull in the fresh air. Since I did this, I havn't cracked 1 rotor. I now have no direct air pointing at the inner face.
The cooling ducts are (sometimes) pointed at the inside face.
Since I realized this, I changed my ducts to point to the inner center hub so the veins can pull in the fresh air. Since I did this, I havn't cracked 1 rotor. I now have no direct air pointing at the inner face.
10-02-2007, 01:58 AM
#14
Team Owner
some of the guys i run with are testing a new system that combines a fan for non ducted cars that blowes air into the center of the hat, and is controled buy a temperature switch mounted to a plate on the spindle, the proposed name ... brake wind!
the point is the fan will run after you pull in the pits and cool the rotor evenly,,,
the point is the fan will run after you pull in the pits and cool the rotor evenly,,,
10-02-2007, 02:02 AM
#16
Safety Car
10-02-2007, 02:46 AM
#18
Team Owner
10-02-2007, 06:53 AM
#19
Burning Brakes
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Take a look at the rotor from the top - besides things already mentioned, the outer face is thinner than the inner face...designed to fail where you can see it.
10-02-2007, 07:30 AM
#20
Drifting
Cracks form at the surface first. However, cracks continue to propagate as long as there is sufficient energy to overcome the material's resistance to crack growth. So, the rotor can eventually become "cracked all the way through" as you observed.
Bill 2
